The present invention is directed to the control of a variable phase mechanism for a camshaft of an internal combustion engine, and more particularly to a method of determining the phase angle of the camshaft relative to an engine crankshaft.
Mechanisms for continuously varying the phase of a camshaft (intake and/or exhaust) relative to the crankshaft for purposes of reducing exhaust gas emissions and improving engine performance are well known in the art of internal combustion engine controls. In general, accurate knowledge of the actual cam phase angle and its rate of change with respect to time are essential to the achievement of accurate phase angle control. The conventional approach to determining cam phase angle involves determining the engine speed, computing a displacement between the camshaft and crankshaft according to the product of engine speed and the time between crankshaft and camshaft position pulses, and then converting the computed displacement to a corresponding phase angle. The cam phase angle velocity, in turn, is obtained by determining a change in cam phase angle, and dividing by the intervening time interval. These calculations are repeatedly performed in synchronism with engine rotation, and obviously create a substantial computational burden for the system controller. Accordingly, what is desired is a more efficient and less burdensome method of determining the cam phase angle and its rate of change.
The present invention is directed to an improved method of determining the phase angle of an internal combustion engine camshaft relative to a crankshaft of the engine, which significantly reduces the computational burden compared to conventional methods. According to the invention, the cam and crankshaft position pulses are identified by corresponding clock values, and the cam phase angle relative to the crankshaft is determined by computing a simple ratio of first and second time intervals based on the clock values, and multiplying the ratio by a known angle. One of the time intervals is defined by a time difference between successive crankshaft and camshaft pulses, and the other time interval is defined by a time difference between successive crankshaft pulses. The known angle is the angle of crankshaft rotation between the successive crankshaft pulses. As with the known method, the cam phase angle velocity is obtained by determining a change in cam phase angle, and dividing by the intervening time interval.